1. Field of the Invention
The present invention relates to motor controls for permanent magnet AC synchronous motors and brushless DC motors.
2. Description of Related Art
In FIG. 1, a known three phase motor drive system for motor PM is depicted with a power inverter and a power source that includes a battery BAT together with capacitor CAP. The power inverter includes primary commutation switches Q1–Q6 together with bypass diodes D1–D6 configured as shown. Permanent magnet motor PM is a three phase motor driven by three stator windings A, B and C. To energize windings A and B with a current in one direction, switches Q1 and Q5 are turned on, and all other switches are turned off. To energize windings A and B with a current in the other direction, switches Q2 and Q4 are turned on, and all other switches are turned off. To energize windings A and C with a current in one direction, switches Q1 and Q6 are turned on, and all other switches are turned off. To energize windings A and C with a current in the other direction, switches Q3 and Q4 are turned on, and all other switches are turned off. To energize windings B and C with a current in one direction, switches Q2 and Q6 are turned on, and all other switches are turned off. To energize windings B and C with a current in the other direction, switches Q3 and Q5 are turned on, and all other switches are turned off.
As discussed in U.S. Pat. No. 6,236,179 to Lawler, et al., incorporated herein by reference, AC synchronous motors and brushless DC motors are controlled through commutation of solid state switching devices connected to their stator windings. These motors can be of the permanent magnet (PM) type in which permanent magnets are used on the rotor instead of rotor windings. As the speed of the rotor increases, the voltage developed in the stator (referred to as the “back emf”) increases. This, in turn, requires that higher and higher terminal voltages be applied to produce the desired torques. Base speed is that speed which is at the top of the constant torque range and at the beginning of the constant horsepower range. In many uses, it is desirable to limit terminal voltage at a certain speed and yet maintain a constant horsepower over a certain speed range above base speed. The ratio of the highest speed that can be attained to the base speed at which the limit of terminal voltage is reached is termed the constant power speed ratio. Attaining a desired constant power speed ratio is made more difficult when the motor inductance is in the microhenry range.
PM motors with interior mounted magnets (IPMs) have been shown to have constant power speed ratios of 7.5:1. However, these types of PM motors are not commercially available.
In traction devices such as electric vehicles, the torque-speed specifications call for a constant torque up to some base speed, and then constant horsepower operation up to a higher speed. PM electric motors with rare earth surface-mounted permanent magnets are viable candidates for such applications due to their power density and efficiency. These motors are electrically commutated and are driven by inverters.
Camber et al., U.S. Pat. No. 5,677,605, issued Oct. 14, 1997, discloses a drive system for a brushless DC motor which uses PWM inverter, and phase timing advancement to control operation in the constant power range above base speed. This patent discloses a three-phase brushless DC motor driven by a six-step PWM drive. The commutation switches include IGBTs (insulated gate bipolar transistors) and MOSFETs (MOS field effect transistors) for the primary switching devices in parallel with bypass diodes.
As speed increases and commutation takes place at a rapid rate, this arrangement may allow for continuous conduction of the phase current and conduction by the bypass diodes at undesirable times, leading to the loss of power and efficiency. The inverter and the motor may heat up, thereby requiring additional cooling measures.
U.S. Pat. No. 6,236,179 issued May 22, 2001 to Lawler et al., the subject matter of which is incorporated herein. FIG. 2 depicts a modification, taught by Lawler et al., of the known system depicted in FIG. 1. A circuit for controlling a three-phase machine PM, that has a stator with three stator windings, includes a controller (as depicted in FIG. 1), three primary commutation switch pairs, a neutral terminal N and three stator winding circuits. The first switch pair includes a first primary commutation switch (Q1, D1) and a second primary commutation switch (Q4, D4) connected at node NA. The second switch pair includes a first primary commutation switch (Q2, D2) and a second primary commutation switch (Q5, D5) connected at node NB. The third switch pair includes a first primary commutation switch (Q3, D3) and a second primary commutation switch (Q6, D6) connected at node NC. The first stator winding circuit includes commutation control switch SWA connected between node NA and one end of winding A of the three-phase motor PM. The other end of winding A is connected to the neutral terminal N. The second stator winding circuit includes commutation control switch SWB connected between node NB and one end of winding B of the three-phase motor PM. The other end of winding B is connected to the neutral terminal N. The third stator winding circuit includes commutation control switch SWC connected between node NC and one end of winding C of the three-phase motor PM. The other end of winding C is connected to the neutral terminal N.
Each of the commutation control switches SWA, SWB and SWC consist of two anti-parallel silicon controlled rectifiers. The silicon control rectifiers (known as SCRs) are non-conducting in reverse bias situation, and are also non-conducting, even when forward biased, unless a trigger signal is received while the SCR is forward biased. Once triggered while forward biased, the SCR becomes conductive. After an SCR is conducting, it remains conducting until the voltage across the SCR drops to zero or the SCR becomes reverse biased. In this way, the controller can provide a trigger pulse to the SCR and thereby initiate the on cycle any time the SCR is forward biased; however, the SCR can be returned to the non-conducting state only at the end of a cycle when the voltage across the SCR reverses. The controller is coupled to the SCRs to control a phase advance conduction angle of the primary commutation switches relative to a point where a supply voltage is equal to the back emf.
The Lawler et al. circuit is applied to three-phase motors with low motor inductance. This circuit is intended for motors that operate at speeds well above the base speed where the base speed is the highest speed at which a specified torque is obtained. For example, the motor may be specified to require XXX torque up to speeds of YYY rpm. As motor speed increases, the stator winding develop a back emf. When the back emf magnitude from a motor winding becomes larger than the supply voltage, some means must be found to further drive the motor since current still needs to be injected into the stator windings of the motor PM in order for the motor to continue to develop power. Lawler et al. drives the motor beyond base speed by injecting current into the stator winding at a motor rotational angle advanced ahead of the angle at the time when the back emf magnitude from a motor winding becomes larger than the supply voltage. Lowler et al. provides this current at an advance angle by using the controller to trigger one of the SCRs in the appropriate commutation control switch, SWA, SWB or SWC at the advance angle. In Lawler, et al, the advance angle is provided in a range from zero to sixty degrees. This advance angle controls the developed power as explained in Lawler, et al. When the voltage across an SCR become zero or negative, the SCR is turned off. At some speeds only slightly greater than base speed and at some advance angles, the Lawler et al. circuit may not result in the outgoing phase current reaching zero before the time that the phase is to be switched back into service. This results in a “commutation failure” which is not catastrophic but which does reduce the average output power and increase the RMS current.